Originally Posted by crankysailor
Regarding pre tension.. well I have heard of folks using winches but I wasn't on the boat when I made them. I basically hung on them with my body weight. Now that you mention it that feels kind of silly but I reasoned it wouldn't be orders of magnitude away from what a winch would pull
... or would it?
Yea, actually probably quite near an order of magnitude . . .150-200lbs vs 2000-3500lbs.
You really really should not cut the tails short until they are loaded up near to working strength. A diamond, which has not been properly pre-tensioned will slip/compress some quite good distance when the load comes on and will suck those short tails into the knot and come untied. We had a (offroad recovery) video showing exactly this here quite recently in another thread.
If you can't or don't want to preload that high, you should just simply leave the tails long (like 4"-6" sticking out of the knot) until they get worked up to near working load (like thru a 35 kt blow). It's not quite as nice looking with long tails but it is not horrible and there is pretty much no functional problem with it.
This is not an idle problem . . . it is the most common failure point of diamond soft shackles - even from 'pro' constructed ones I have tested . . . people just don't tension them enough and then cut the tails super short, and they then suck the tail in and come untied way way short of the strength they should/could have.
Back when we were developping/refining soft shackles on that old sailing anarchy thread . . . there was a debate about what strength we should cite for them. I argued we should cite the strength of a 'well made one'. While another participant said we should cite the strength of 'a typical diy
job'. And the difference is very roughly double.